1. Field of the Invention
The present invention relates to a nuclear magnetic resonance (NMR) imaging apparatus and, more particularly, to a tuning device for tuning a transmitter coil and/or a receiver coil to a resonant frequency in the NMR imaging apparatus, and an NMR imaging apparatus including the tuning device.
2. Description of the Prior Art
In a conventional NMR imaging apparatus, an object to be examined is placed in a homogeneous and static magnetic field. A sequence of time-dependent magnetic fields, so-called RF pulses, with the frequency equal to the Larmor frequency of specific atomic nuclei to be excited is then applied to this object by an RF transmitter coil. Following this application of the RF pulses, the excited nuclei go through a relaxation process. This process is detected as a NMR signal by a receiver coil. Throughout this excitation and relaxation, various magnetic field gradients are also applied to the object by gradient coils. These magnetic filed gradients enable the examiner to determine the locations of the nuclei upon analysis of the NMR signals. This procedure is repeated with different magnetic field gradients, and the NMR signals thus accumulated are analyzed to obtain information on density distributions and the relaxation times of the nuclei. This information is utilized subsequently to construct tomographic images according to various computational schemes.
In such a NMR imaging apparatus, the RF transmitter coil most commonly is comprised of a pair of saddle-shaped coils. It is usually equipped with a tuner including a variable capacitor, so as to form the resonant LC circuit together for the sake of effective emission of the RF pulses.
The variable capacitor is necessary in order to deal with the different resonant frequencies for different objects to be examined due to the existence of stray capacitance between the RF transmitter coil and the object.
Meanwhile, the RF transmitter coil is required to generate powerful RF pulses in order to induce a sufficient number of NMR excitations. This, in turn, requires very high voltages to be applied to the variable capacitor of the tuner. The variable capacitor therefore needs to be capable of enduring high-voltage conditions.
For this reason, the variable capacitor of the tuner is of the heavy-duty type, and for adjusting the capacitance of this variable capacitor, a powerful adjusting means is required.
Furthermore, this adjusting means has to be capable of functioning in a strong magnetic field. This is because the adjusting means is to be placed in a strong magnetic field along with the tuner. Consequently the usual electric motor cannot be used because it does not operate properly in the presence of a magnetic field.
The adjusting means satisfying these requirements had in the past been either a manually operated type or a permanent magnet motor type having its permanent magnet removed, with the surrounding magnetic field substituting for the role of the permanent magnet.
However, in the former type, the manual operation is tedious, time-consuming and liable to mistakes, while in the latter type, the motor tends to disturb the homogeneity of the magnetic field and generate electric noises.
To cope with these problems, an adjusting means comprising a fluid motor had been proposed by the present inventor, as disclosed in Japanese Patent Laid-Open Specification No. 62-59845.
This fluid motor adjusting means has been able to eliminate the problems of the other adjusting means described above, and it has the added capability of being controlled automatically.
However, the fluid motor adjusting means has a limited precision and a delayed response due to the compressibility of fluids; i.e., a time-lag between the actual operation of the motor and the control signaling for the transmitter tuning exists because of the time required for compressing the fluid, and this makes the precision of the tuning limited.
In addition, the fluid motor generates large acoustic noises when the fluid is a highly compressible medium such as air, and the leakage can occur when the fluid is oil.